The invention relates to processes for the establishment of a communication using video telephony between a calling terminal and a called terminal via public or private networks, as well as video telephony terminals permitting the implementation of such a process.
It finds a particularly significant application in the telephone installations having at least one videophone screened by an acceptance telephone set, the call from the videophone to a number available to third parties, which will be referred to hereinbelow as "public", giving rise to the direction of the incoming call to the screening telephone set or "screening" set.
The conventional architectures of installation, where the screening set is a telephone and not a videophone, prohibit the establishment of the communication on an incoming call from the network outside the periods in which the screening is eliminated, where the called terminal has a conventional constitution.
A reminder will be given, first of all, of the manner in which a call from a calling videophone to a called videophone is normally established.
The calling subscriber selects the number of his partner and commands the sending of the videophone call message. The calling videophone then attempts to establish a link onto an ISDN network via a first channel B in TBCC (Transparent B Circuit Communication) mode. If the communication is accepted by the destination videophone, a possible second B channel in TBCC mode is established, if the videophones require this. The videophone communication can then take place, for example with utilization of the protocols and especially of the frame H221.
If the first transparent B channel cannot be established, because the destination set is not a videophone terminal but an ordinary telephone set, the calling videophone terminal establishes a nontransparent channel NTBCC and reverts to traditional telephony, after having cleared the first TBCC if it had already been established.
If the called videophone terminal is screened by a telephone set, the calling videophone terminal will not be able to establish a communication of videophone type and will revert to telephone mode.
The Patent Application FR No. 91 10,493 proposes a process permitting this impossibility to be set aside; according to this process, once a telephone communication has been established between the terminals, on an NTBCC channel, a service message is sent from the called terminal, which service message is included in a message to clear the NTBCC line and contains a secret number for (unscreened) direct calling of the called videophone and there is established directly, from the calling terminal, a link on two type B channels by utilizing the unscreened call secret number.
This process gives very good results. However, it demands that not only the called terminal but also the calling terminal be adapted.